1. Field of the Invention
The invention relates to diazo compositions which comprise, as the yellow coupling component, a 2-alkyl-4-aralkyl resorcinol, and to diazotype materials prepared therefrom.
2. Description of the Prior Art
It has long been known that diazonium compounds are capable of reacting with compounds such as aromatic amines, phenols, phenol ethers, compounds containing active methylene groups, and the like, to form colored reaction products known as azo dyes. This "coupling" reaction has proven to be most useful in a number of commercial applications, such as in the preparation of textile dyestuffs, and in diazo imaging systems.
Two of the most accepted types of diazo imaging processes employ a diazotype material which comprises a light-sensitive diazonium compound coated on a base or a support material; and, depending on whether or not the diazotype material is designed for use in a one-component or semi-wet development process, or a two-component or "dry" process, the layer or coating containing the light-sensitive diazonium compound may or may not also contain a coupling component for the diazonium compound.
In the case wherein the diazonium compound is present on the base material without a coupling component (i.e., in the case of a one-component or semi-wet development diazotype material), the light-sensitive diazotype material is exposed, and is then developed by applying a developing solution containing a coupling component to the exposed diazotype material. During the exposure step, the light-sensitive diazonium compound is destroyed or altered by the light striking the exposed areas, thereby making the diazonium compound unavailable for coupling with the coupling component in the development step. The subsequent treatment of the exposed diazotype material with the developing solution containing the coupling component results in the formation of an azo dye image in those areas wherein the unaltered diazonium compound is still available for coupling with the coupling component.
In the case wherein the diazonium compound is present on the base material along with a coupling component or components (i.e., in the case of a two-component or dry development diazotype material), the light-sensitive diazotype material is exposed, and is then developed by subjecting the exposed diazotype material to an alkaline atmosphere. As in the case with the one-component diazotype process, the light-sensitive diazonium compound is destroyed or altered by light in the exposed areas during the exposure step, thereby making it unavailable for coupling with the coupling component or components which are present in the diazotype material. When the exposed diazotype material is then subjected to an alkaline atmosphere, such as ammonia, the alkaline conditions permit the coupling reaction to take place between the coupling component and the unaltered diazonium compound to form the colored azo dye image.
Although the color of the azo dye image which is obtained in any given instance depends primarily on the coupling components and the diazonium compounds which are employed, coupling components are often generally described as being couplers of a given color--the color being the color of the dye which is usually obtained when the particular coupler in question couples with a diazonium compound. For example, couplers such as monohydric phenols, catechols, catechol derivatives, resorcinols, resorcinol derivatives, diketones, acetoacetic acid derivatives, acetonitriles, cyanacetylamides and the like, usually result in yellow, orange, sepia, brown, red or maroon azo dyes. Thus, couplers from such classes of materials are conveniently referred to as yellow, orange, sepia, brown, red, or maroon couplers. On the other hand, couplers such as naphthoic acid derivatives, dioxynaphthalene derivatives, pyronones, hydroxypyronones, and the like, usually result in blue or violet azo dyes, and thus are conveniently referred to as blue or violet couplers.
One group of highly useful coupling components are the yellow couplers, since the dyes obtained from these couplers usually have actinic adsorption characteristics which permit their use as the sole coupler is a diazo composition which is employed to prepare diazotype "masters" or intermediates, and since couplers from this group can often be employed as shading components when used in conjunction with another coupler or couplers. As indicated above, compounds containing active methylene groups, compounds such as acetonitriles, derivatives of acetonitriles, and the like, have been employed as yellow couplers in diazo compositions, (cf., for example, U.S. Pats. Nos. 1,989,065; 2,531,004; 2,537,001 and 2,537,106); yet a number of these active-methylene types of couplers have exhibited a tendency, when employed in two-component diazo compositions, to precouple with the diazonium compound which is present in said compositions during storage even in the presence of the stabilizers which are usually employed. This tendency to precouple prior to exposure and development has limited the use of these materials somewhat, since even a slight amount of precoupling can result in the formation of an azo dye in those areas of the diazotype material which are the background or "cleared" areas of the diazotype print. In addition to this tendency to precouple, a number of these prior-art, active-methylene types of couplers also result, upon coupling, in dyes which have an undesirable reddish hue and/or which have a tendency to fade upon subsequent exposure to light.
It should be apparent from the above, that, in addition to obtaining a single-color azo dye image, one should be able to obtain a mixture of azo dyes (and thus a mixture of colors) by including more than one coupling component or more than one diazonium compound in the light-sensitive diazo composition. Thus, by a proper choice of coupling components and/or diazonium compounds, one should be able to obtain a wide variety of colors in the resulting azo dyes, including black. However, the achievement of a uniform color over a wide range of image densities from a diazo composition containing more than one coupling component has proven difficult to obtain in actual practice. In order to obtain a uniform color over a wide range of image densities, it is essential that the coupling activity of the various coupling components with diazonium compound or compounds which are employed be carefully matched, and that the combined absorptions of the azo dyes produced from the various couplers cover the entire visible spectrum. It is also essential that none of the azo dyes produced from the various coupling components be subject to a "color-shift" or change of shade due to a change in pH, else the resulting dye image of the diazotype material may shift from the neutral point.
Thus, although resorcinol sulfides, resorcinol sulfoxides and diresorcinols are several classes of yellow azo coupling components which exhibit excellent resistance to subsequent fading to light, unfortunately these particular classes of yellow coupling components exhibit a severe tendency to color-shift when incorporated into black-line formulations. Similarly, AON's such as 1-hydroxy-2-naphthoic acid piperidide also exhibit a severe tendency to color-shift when incorporated into black-line formulations.
Attempts have also been made to employ alkyl substituted resorcinols (e.g., 2-alkyl resorcinols) and dialkyl substituted resorcinols (e.g., 2,4-dialkyl resorcinols) in blackline formulations. Although the diazotype materials resulting from such formulations exhibit little or no tendency to color-shift, these particular classes of resorcinol coupling components have a relatively slow rate of coupling and are readily oxidized to colored oxidation products.